In short
Bacteriostatic water (BAC water) is sterile water containing 0.9% benzyl alcohol, a preservative that suppresses bacterial growth. This allows repeated withdrawals from a single vial for up to 28 days without the solution being compromised. It is the standard solvent for reconstituting lyophilized research peptides. What separates it from plain sterile water is precisely that preservative, which turns a single use solvent into a multi dose one.
- Composition: highly purified sterile water + 0.9% benzyl alcohol (preservative)
- Purpose: dissolving (reconstituting) lyophilized peptides for laboratory work
- Main advantage: repeated withdrawal from one vial for up to 28 days after first puncture
- Storage: room temperature away from light, refrigerated once opened
- Where to buy: from a research chemical supplier that documents each batch, bacteriostatic water 10 ml
| Parameter | Bacteriostatic water | Sterile water (WFI) | Saline (0.9% NaCl) |
|---|---|---|---|
| Preservative | 0.9% benzyl alcohol | none | none |
| Use | multi dose (days to weeks) | single use | single use |
| Suitable for peptides | yes, the standard | yes, but single use only | conditionally, may interfere |
| Shelf life after opening | up to 28 days | use immediately | use immediately |
This page is intended strictly for research and laboratory purposes (RUO). It is not medical or administration guidance.
What is bacteriostatic water
Bacteriostatic water is highly purified sterile water to which benzyl alcohol has been added at a concentration of 0.9%. Benzyl alcohol acts bacteriostatically, meaning it does not kill bacteria outright but reliably suppresses their growth and reproduction. That is the key difference from ordinary sterile water.
In practice it solves one specific problem. Research peptides are supplied in lyophilized (freeze dried) form and must be dissolved in liquid before use. If you used an unpreserved solvent, then after the first puncture of the rubber stopper and withdrawal of part of the solution, microorganisms from the air could enter the remainder through that same opening. Over several days of gradual withdrawal the solution would become contaminated. Benzyl alcohol prevents this and makes repeated withdrawal from a single vial possible. A technical breakdown of the composition sits on the product page for bacteriostatic water.
Bacteriostatic water vs sterile water vs saline
The decisive difference lies in the preservative and therefore in how many times the solvent can be used. Bacteriostatic water contains benzyl alcohol and is multi dose, while sterile water for injection (WFI) and saline contain no preservative and are intended for single use. For reconstituting peptides you will draw from gradually, bacteriostatic water is therefore the standard.
- Bacteriostatic water (BAC water): contains 0.9% benzyl alcohol, neutral pH, suitable for repeated withdrawal. The most common choice for peptides.
- Sterile water for injection (WFI): pure water with no preservative. Sterile only until first opening, after which immediate use is required. Used where a preservative would interfere, or for single batch preparation.
- Saline (0.9% NaCl): isotonic, but contains sodium and chloride ions that can affect stability or solubility in some sensitive peptides. Also preservative free.
A solvent by solvent comparison with practical notes for specific peptides is covered in the guide on how to reconstitute a lyophilized peptide.
Why exactly 0.9% benzyl alcohol
The 0.9% concentration is a compromise validated by decades of practice, balancing sufficient preservative efficacy against gentleness toward the peptide. A lower proportion might not reliably suppress bacterial growth across the working life of the vial, while a higher one could affect the stability of sensitive molecules.
Benzyl alcohol is one of the two most common preservatives used in peptide and protein preparations (Meyer 2007, review of preservatives in parenteral products). Its antimicrobial action derives from its lipophilic nature, incorporation into the bacterial membrane and disruption of its permeability (Karabit 1986; Fuller 1990, phenolic and aromatic alcohols). At the same time, benzyl alcohol can under certain conditions influence protein aggregation in reconstituted lyophilized formulations, which is exactly why the concentration is kept low at 0.9% (Thirumangalathu 2006).
How long bacteriostatic water lasts once opened
The standard recommendation is to use bacteriostatic water within 28 days of the first stopper puncture. Up to that point the preservative reliably suppresses microbial growth. After 28 days the remainder should be discarded even if it looks clear, because preservative protection declines over time.
An unopened vial keeps until the manufacturer’s stated date, typically years, provided it is stored correctly. The deciding moment is therefore the first withdrawal, and the 28 days are counted from there. For good order, write the opening date directly on the vial.
How to store bacteriostatic water
Store unopened bacteriostatic water at room temperature away from direct light, keep it refrigerated (2 to 8 °C) once opened, and use it within 28 days. Cold and darkness slow any chemical change and keep the solution in optimal condition.
Avoid freezing, repeated temperature swings, and storing the vial in the refrigerator door where temperature fluctuates most. Always close the vial properly and handle it cleanly, drawing each time with a sterile needle after disinfecting the stopper.
What bacteriostatic water is used for in peptide work
Bacteriostatic water is the most common solvent for reconstitution, meaning the return of a dry lyophilized peptide back into solution. It is added to the vial containing the powder, the peptide dissolves gently, and the result is a solution of defined concentration that can be worked with further.
The procedure itself has rules: the solvent is applied slowly against the vial wall, never directly onto the lyophilizate, and the vial is swirled gently rather than shaken. The full step by step, including concentration calculation and storage of the finished solution, is covered in how to reconstitute a lyophilized peptide. The compounds themselves are listed in the research peptides catalogue.
Which vial size, 5 ml or 10 ml
For ongoing laboratory work 10 ml is the more practical size, because it covers the reconstitution of several peptide vials while still being consumable inside the 28 day window after opening. Smaller 5 ml vials make sense when you work only occasionally and a larger volume would expire unused.
The decision reduces to simple arithmetic. If a single peptide vial typically takes 1 to 3 ml of solvent, a 10 ml vial covers roughly three to ten reconstitutions. The constraint is that they must all happen within 28 days of the first puncture, otherwise the remainder is wasted. Molequa supplies the 10 ml format, which suits continuous work rather than a one off experiment.
Where to buy bacteriostatic water
Bacteriostatic water for research use is best sourced from a research chemical supplier that provides documentation for every batch and guarantees both sterility and preservative concentration. That is the difference from an undocumented purchase of unknown provenance.
When choosing, check three things: the declared benzyl alcohol concentration (it should be exactly 0.9%), confirmed batch sterility, and clear storage and shipping conditions within the EU. Molequa supplies bacteriostatic water 10 ml with documentation and dispatch from an EU warehouse, strictly for research and laboratory purposes.
Safety and what to watch for
Bacteriostatic water is a laboratory chemical, not a medicine, and is intended solely for research use. Benzyl alcohol as a preservative has a well documented safety profile in the laboratory context, but there are populations in which preparations containing benzyl alcohol are generally avoided.
The literature documents, for example, a risk in neonates, where benzyl alcohol was linked to serious toxicity known as gasping syndrome (Gershanik 1982; Brown 1982). This is one reason preserved solutions are not universal, and why bacteriostatic water is handled as research material under laboratory rules rather than as a consumer product.
Frequently asked questions about bacteriostatic water
What is bacteriostatic water?
Sterile purified water containing 0.9% benzyl alcohol, which suppresses bacterial growth. It is used as the standard solvent for reconstituting lyophilized research peptides because it permits repeated withdrawal from a single vial.
What is the difference between bacteriostatic and sterile water?
Bacteriostatic water contains a preservative (benzyl alcohol) and is multi dose, allowing withdrawals for up to 28 days. Sterile water for injection has no preservative and is single use, requiring immediate consumption after opening.
How long does bacteriostatic water last after opening?
The standard recommendation is to use it within 28 days of the first stopper puncture. An unopened vial keeps until the manufacturer’s date, typically years, when stored correctly.
Why does bacteriostatic water contain 0.9% benzyl alcohol?
It is a validated compromise between reliable preservative protection and gentleness toward sensitive peptides. A higher concentration could affect molecular stability, a lower one might not suppress bacterial growth sufficiently.
Can I use saline instead of bacteriostatic water?
For a single batch preparation sometimes yes, but saline contains ions that can affect solubility or stability in some sensitive peptides, and it has no preservative. For repeated withdrawal, bacteriostatic water is the standard.
Is 5 ml or 10 ml the better size?
For continuous laboratory work 10 ml, since it covers roughly three to ten reconstitutions and can still be used within 28 days of opening. A 5 ml vial suits infrequent work where a larger volume would go to waste.
Where can I buy bacteriostatic water in the EU?
From a research chemical supplier that documents each batch, guarantees sterility and preservative concentration, and ships within the EU. For example bacteriostatic water 10 ml from Molequa.
How should bacteriostatic water be stored?
Unopened at room temperature away from light, opened in the refrigerator (2 to 8 °C), and used within 28 days. Do not freeze and avoid temperature swings.
Sources and citations
- Meyer BK, et al. Antimicrobial preservative use in parenteral products: past and present. J Pharm Sci 2007. PubMed 17722087
- Karabit MS, et al. Studies on the evaluation of preservative efficacy: antimicrobial characteristics of benzylalcohol. 1986. PubMed 3531242
- Fuller SJ, et al. Antibacterial activity of phenolic compounds and aromatic alcohols. 1990. PubMed 1697976
- Thirumangalathu R, et al. Effects of benzyl alcohol on aggregation of recombinant human IL-1 receptor antagonist in reconstituted lyophilized formulations. 2006. PubMed 15614819
- Gershanik J, et al. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med 1982. PubMed 7124703
- Brown WJ, et al. Fatal benzyl alcohol poisoning in neonatal intensive care units. 1982. PubMed 7133084
Legal notice
All information on this page is provided strictly for research and educational purposes. Bacteriostatic water and research peptides are intended solely for laboratory research use (research use only). They are not medicines, dietary supplements or cosmetics, and are not intended to diagnose, treat or prevent any disease in humans or animals. This is not medical or administration guidance.
